SLIDE 1
The Exotic Wave in 190 GeV p p at COMPASS Tobias Schl - - PowerPoint PPT Presentation
The Exotic Wave in 190 GeV p p at COMPASS Tobias Schl - - PowerPoint PPT Presentation
The Exotic Wave in 190 GeV p p at COMPASS Tobias Schl uter for the COMPASS collaboration Ludwig-Maximilians-Universit at M unchen June 16, 2011 in diffractive scattering Possible quantum
SLIDE 2
SLIDE 3
πη′ in diffractive scattering
Possible quantum numbers for the πη′ system: L S-wave P-wave D-wave F-wave G-wave · · · JPC 0++ 1−+ 2++ 3−+ 4++ · · · Hence: P-wave resonant → exotic meson. This system has been studied by the following experiments: experiment beam momentum reaction year published VES 37 GeV/c π−N → η′π−N 1993, 2005 E852 18 GeV/c π−p → η′π−p 2001 COMPASS 190 GeV/c π−p → η′π−p 2012 (?) They all see a very strong P-wave.
SLIDE 4
Previous πη′ results – VES
Results from VES (Be target, 37 GeV):
◮ VES sees the a2(1320) (peak in D+-wave) ◮ VES says: “there may be an a2(1700)”
explaining the broad structure in the D+-wave
◮ VES says: “there may be an exotic
π1(1600)” Note the jump in the relative P+ − D+ phase near 2 GeV
SLIDE 5
Previous πη′ results – BNL E852
Results from BNL E852 (proton target, 18 GeV):
◮ they see the a2(1320) (peak in D+-wave) ◮ they add a G+-wave to the fit, gives: a4(2040) ◮ they explain the broad D+-wave with an a2(1700) and the P+-wave
with an exotic resonance π1(1600)
◮ they find an unusual t-slope
Note the various jumps at 2 GeV
SLIDE 6
Remarks on PWA formalism
For a given mass, two-body states in the reflectivity basis (ǫ, ℓ, m) have the form (θ, φ Gottfried-Jackson angles) Aǫlm ∝ Y m
ℓ (θ, 0)
- sin(mφ)
ǫ = +1 (m > 0) cos(mφ) ǫ = −1 (m ≥ 0) The observed intensity is then proportional to I(θ, φ) =
- lm
T+1,ℓmA+1,ℓm(θ, φ)
- 2
+
- ℓm
T−1,ℓmA−1,ℓm(θ, φ)
- 2
where the production amplitudes Tǫℓm were introduced. Important observations: only negative reflectivity (= unnatural exchange) contributes to intensity at φ = 0; all positive reflectivity waves with m = 1 have the same φ dependency. Negative reflectivity compatible with zero in VES, BNL, COMPASS analyses.
SLIDE 7
The COMPASS spectrometer
◮ fixed target experiment at CERN’s SPS accelerator ◮ variety of beams available (pos/neg muon, pos/neg hadron) ◮ variety of targets (polarized targets, LH2, solid state) ◮ diverse physics program ◮ 2008, 2009 : hadron beam runs with various targets
In this talk: 2008 data, negative pion beam at 191 GeV, LH2 target
SLIDE 8
Data selection
Final state selected: exclusive 3 tracks, 2 photons
) γ γ m( 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 entries / 0.5 MeV 20 40 60 80 100
3
10 × COMPASS 2008 p γ γ
+
π
- π
- π
→ p
- π
preliminary
) [GeV] η
+
π
- π
m( 1 1.5 2 2.5 3 3.5 4 4.5 entries [MeV] (2 entries per event) 2 4 6 8 10 12
3
10 × COMPASS 2008 p η
+
π
- π
- π
→ p
- π
w/o acceptance correction
preliminary
◮ select exclusive events with 3 tracks + recoil proton, 2 good ECAL
clusters
◮ select η → 2γ (left) ◮ select η′ → π−π+η (right)
SLIDE 9
Data selection
Final state selected: exclusive 3 tracks, 2 photons
) γ γ m( 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 entries / 0.5 MeV 20 40 60 80 100
3
10 × COMPASS 2008 p γ γ
+
π
- π
- π
→ p
- π
preliminary
) [GeV] η
+
π
- π
m( 1 1.5 2 2.5 3 3.5 4 4.5 entries [MeV] (2 entries per event) 2 4 6 8 10 12
3
10 × COMPASS 2008 p η
+
π
- π
- π
→ p
- π
w/o acceptance correction
preliminary
’) [GeV] η π m( 1.5 2 2.5 3 3.5 4 4.5 5 entries / 20 MeV 100 200 300 400 500 600 COMPASS 2008 )p γ γ
+
π
- π
’( η
- π
→ p
- π
w/o acceptance correction
preliminary Result:
◮ 18 000 events with
m(η′π) < 2 GeV/c2, 35 000 total
◮ mass reach beyond 2 GeV/c2 ◮ additionally, about 3 000 events
in πη′, η → 3π channel
SLIDE 10
First look at the data: t slopes
We find indication for a continuous transition between different production mechanisms, fitting the t distribution in several areas mass bin fit with A exp(−B|t|) fit with A|t| exp(−B|t|) m < 1.5 5.5 8.2 1.5 < m < 1.9 5.1 7.5 1.9 < m < 2.2 4.8 7.1 2.2 < m < 3 4.6 6.9 (BNL fitted with a simple exponential between 0.25 < |t| < 1.0 GeV/c2, they found B = 2.93/GeV2) We find: higher mass → broader slope and: clear contradicition with BNL
SLIDE 11
Input to the PWA
Like previous analyses, we used all waves with ℓ ≤ 2, m ≤ 1 and additionally the ǫ = +1, ℓ = 4, m = 1. I.e.: ǫ = +1 P+ D+ G+ ǫ = −1 S0 P0 P− D0 D− As expected from Pomeron (i.e., natural) exchange, the negative reflectivity waves turn out compatible with zero (below 2 GeV). Ambiguities are bounded by the size of the negative waves, i.e. they are not a problem.
SLIDE 12
PWA results – P+ and G+ waves
m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
2
events / 40 MeV/c 1000 2000 3000 4000 5000 6000 7000 8000
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
) intensity
- +
= 1
PC
(J
+
P m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 ) [deg]
+
/ P
+
arg(D
- 180
- 160
- 140
- 120
- 100
- 80
- 60
- 40
- 20
20
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
+
- P
+
phase D m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
2
events / 40 MeV/c 1000 2000 3000 4000 5000 6000
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
) intensity
++
= 2
PC
(J
+
D
Intensity P+ Phase D+ − P+ Intensity D+ Clear phase-motion from a2(1320), jump in phase near 2 GeV, slow phase-motion in range of P+-wave intensity peak.
SLIDE 13
PWA results – can the G+-wave clarify the picture?
m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
2
events / 40 MeV/c 1000 2000 3000 4000 5000 6000 7000 8000
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
) intensity
- +
= 1
PC
(J
+
P m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 ) [deg]
+
/ P
+
arg(D
- 180
- 160
- 140
- 120
- 100
- 80
- 60
- 40
- 20
20
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
+
- P
+
phase D m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
2
events / 40 MeV/c 1000 2000 3000 4000 5000 6000
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
) intensity
++
= 2
PC
(J
+
D m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 ) [deg]
+
/ P
+
arg(G 300 400 500 600 700 800
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
+
- P
+
phase G m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 ) [deg]
+
/ D
+
arg(G 50 100 150 200 250 300 350 400
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
+
- D
+
phase G m [GeV] 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
2
events / 40 MeV/c 200 400 600 800 1000 1200 1400 1600
preliminary
COMPASS 2008 ’p η
- π
→ p
- π
) intensity
++
= 4
PC
(J
+
G
- Int. P+
∆Φ(D+ − P+) ∆Φ(G+ − P+)
- Int. D+
∆Φ(G+ − D+)
- Int. G+
Clear phase-motion in G+-wave relative to D+ wave, compatible with a4(2040). Again: jump at 2 GeV in phase relative to P+ wave. But: unlike between P+ and G+ no rapid phase jump between D+ and G+ waves at 2 GeV
SLIDE 14
Transition between different production processes?
m [GeV] 1.5 2 2.5 3 3.5 4 4.5 5
GJ
θ cos
- 1
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 5 10 15 20 25 30 35 40 COMPASS 2008 ’p η
- π
→ p
- π
w/o acceptance correction
preliminary
Depicted: cos θGJ of the η′ in the π−η′ GJ restframe vs. m(πη′). Low masses show P and D wave interference, a4 near 2 GeV/c2, above that strong forward/backward peaking indicative of central production. Question: How does the forward/backward peaking at high masses affect the interpretation at low masses?
SLIDE 15
Comparison to ηπ
We also selected the ηπ− final state along the same lines. No PWA yet, for comparison, here’s the same plot as on the previous slide, but for the πη:
m [GeV] 1 1.5 2 2.5 3 3.5 4 4.5
GJ
θ cos
- 1
- 0.8
- 0.6
- 0.4
- 0.2
0.2 0.4 0.6 0.8 1 COMPASS 2008 p η
- π
→ p
- π
w/o acceptance correction
preliminary
Depicted: cos θGJ of the η in the π−η GJ restframe vs. m(πη). Dominated by a2(1320), structures due to a4(2040) visible, again forward/backward peaking at high masses.
SLIDE 16
Conclusions
◮ COMPASS can confirm previous observations of a strong P-wave in
η′π and in addition finds confirming evidence for the a4(2040) → π−η′
◮ the t distribution shows a decreasing slope with increasing m(η′π−)
and the slope disagrees with the findings of E852
◮ resonant interpretation of the P-wave cannot be confirmed (at this
point) The road ahead:
◮ Primary Objective: clarify what happens in the transition between
the regimes below and above ≈ 2 GeV/c2
◮ Secondary Objective: use this to gain clearer understanding of the